The invention relates to an apparatus for the production of multi-layer coatings at reduced pressure.
UK patent specification No. 1,545,897 discloses semi-conductor devices, useful as photovoltaic cells, in which a substrate coated with a coating comprising a layer of undoped amorphous silicon (an intrinsic layer) produced by glow discharge is sandwiched between thinner layers of n-doped and p-doped amorphous silicon also produced by glow discharge. In the production of such cells, it is important, in order to maximise the efficiency of the cells, to avoid any contamination of any one silicon layer with a dopant used to produce another layer. It has therefore been proposed that the different silicon layers should be deposited in separate chambers to reduce the risk of cross-contamination, and an apparatus comprising a line of consecutive separate vacuum chambers has been proposed for the mass production of such cells. Unfortunately the productivity of an in-line apparatus is limited by the time taken to deposit the thicker intrinsic layers, and if similar sized chambers are provided for the deposition of each of the layers, the chambers for deposition of the n-doped and p-doped layers are under-utilised.
This problem is alleviated in the apparatus described in European patent specification No. 60,651; the apparatus comprises a line of 3 separate deposition chambers through which a continuous ribbon of metal foil to be coated is advanced. By making the lengths of the separate deposition chambers approximately proportional to the deposition times required for the different coating layers, the overall efficiency of the apparatus may be improved. However, this involves the use of a long and expensive vacuum chamber for the intrinsic layer, and a continuous process increases the risk of cross-contamination between the deposition chambers.